This invention relates generally to electronic temperature control systems and more particularly to electronic control systems for controlling relay operation in response to inputs from a thermostat. The relays in turn control a fan motor and heat generators such as electric heating banks.
In one prior art system currently in use, a fan motor and a plurality of banks of heater elements are driven through relays under the control of a microprocessor in response to low voltage AC input signals W1, W2, Y and G from a wall thermostat where G is the continuous fan signal, Y is the heat pump or air conditioner compressor signal, and W1 and W2 are the first and second stage electric heat signals, respectively. Such heat thermostats require a minimum load on W1 of 100 mA which is normally dissipated through power resistors on the furnace control board resulting in dissipation of at least 2.4 watts of heat. The dissipation of this heat is facilitated by increasing the surface area of the resistors by placing a plurality of resistors in parallel circuit arrangement between the W1 line and ground to draw the minimum required current. Although this prior art system functions effectively, the 2.4 watts represent wasted energy as well as requiring a special arrangement to allow for heat dissipation.